Bayamon is Puerto Rico's second largest city, sprawling across the metropolitan area just southwest of San Juan. It functions primarily as a residential and commercial suburb, but it has its own identity and a handful of attractions worth seeing if you are based in the San Juan area. The Parque de las Ciencias Luis A. Ferré is a large science park with a small zoo, a planetarium, and open-air exhibits spread across pleasant grounds -- it is particularly good for families with children.
The historic center around the Plaza de Recreo has been spruced up and has a few colonial-era buildings, including the Casa Alcaldía (with its distinctive overhead bridge spanning the street) and the Invención de la Santa Cruz parish church. Two small but worthwhile museums sit within a few blocks: the Casa Museo José Celso Barbosa honours the pioneering Afro-Puerto Rican physician and politician, while the Dr. Agustín Stahl House covers the 19th-century naturalist of the same name.
The Tren Urbano connects Bayamon to San Juan, making it easy to visit without a car. The city is more suburban sprawl than charming old town, but the people are friendly, prices are lower than in tourist-heavy San Juan, and the local food scene is solid -- look for lechoneras and roadside kiosks serving comida criolla. Bayamón is also famous for chicharrón, fried pork rind sold from roadside stalls especially along the Carretera La Cuchilla.